1. Field of the Invention
The present invention relates to a projection system for automatic focus detection and, more particularly, to a projection system for automatic focus detection, which broadens the illumination range and improves focus detection precision in passive and active focus detection schemes suitably used in focus detection executed by a photographing system by projecting a projection pattern from a projection system toward the object side and receiving the projection pattern image reflected by the object side, and is used together with an automatic focus detection device of, e.g., a TTL (Through the Lens) type camera.
2. Related Background Art
Conventionally, upon execution of focus detection by focus detection devices in TTL type cameras such as still cameras, movie cameras, video cameras, and the like, for example, when an object has a low luminance or low contrast, the object is illuminated with light emitted by an illumination device (projection system) built in a camera main body or a flash device, or a projection lens and a predetermined projection pattern are assembled in the illumination device, and the projection pattern is projected onto the object to give contrast to it.
FIG. 11 is a schematic view showing the principal part of a conventional projection system for automatic focus detection. Referring to FIG. 11, a light source 84 comprises, e.g., an LED. A projection pattern 83 has a plurality of light-shielding portions 3a as linear bands and a plurality of light-transmitting portions 3b, as shown in, e.g., FIG. 10. A projection lens 82 is arranged in front of the pattern 83.
In FIG. 11, the projection pattern 83 illuminated by the light source 84 is projected toward the object side via the projection lens 82, and a projection pattern image reflected by the object side is detected by a detection system (not shown), thus attaining focus detection of the photographing system.
FIG. 12 is an explanatory view showing the projection irradiation (illumination) range and the projection pattern image when the projection pattern 83 is projected toward the object side.
In FIG. 12, the interior of a dotted circle 91 corresponds to the illumination range, and the ranges indicated by hatched portions 92 correspond to the projection pattern image on the object obtained by projecting the projection pattern 83 by the projection lens 82.
In most focus detection devices using such automatic focus detection projection system, the focus detection range is located at the center of the photographing field angle (at the center of the optical axis of the photographing lens), and auxiliary light illuminates only the center of the object side except for correction of parallax or the like.
However, in a photographing apparatus having a focus detection region at a point other than the center of the optical axis of the photographing lens, the focus detection region on the object side differs depending on the focal length of the photographing lens. For example, in a lens with a relatively large focal length, the focus detection region at a point other than the center of the optical axis of the photographing lens is close to the center of the photographing range on the object side. However, as the focal length of the photographing lens becomes smaller, the focus detection region at a point other than the center of the optical axis of the photographing lens separates farther outwardly from the photographing range on the object side. That is, the focus detection region is present at a position in a direction separating from the center of the optical axis.
Accordingly, in a camera that can use exchangeable photographing lenses or in a camera which mounts a lens having a variable focal length, the automatic focus detection projection system must illuminate a broader range. For this reason, a conventional projection system uses a plurality of illumination devices, a plurality of light sources, or the like to broaden the illumination range.
However, using a plurality of illumination devices, a plurality of light sources, or the like results in a large illumination device as a whole and high cost.
In recent automatic focus detection devices, it is strongly demanded to attain focus detection at points other than the center of the field angle, and the focus detection range of a camera tends to cover multiple points and a broader range.
FIG. 13 is a schematic view showing the principal part of a projection system for automatic focus detection used together with such automatic focus detection device.
Referring to FIG. 13, a light source unit 104 has a plurality of light-emitting portions (light sources). A projection pattern 103 has an elongated shape in the widthwise direction of the light source unit 104. A projection lens 102 has wide field angle performance.
When a broad projection range (illumination range) is obtained by arranging the plurality of light-emitting portions and illuminating the broad projection pattern 103, an increase in the number of light sources leads to a large device as a whole and high cost, and it is difficult to attain a wide field angle of the projection lens. For this reason, such system can hardly be put into practical applications.
FIG. 14 is a schematic view similarly showing the principal part of a projection system for automatic focus detection used together with such automatic focus detection device. The same reference numerals in FIG. 14 denote the same parts as in FIG. 11.
Referring to FIG. 14, a projection lens 112 is divided into three regions (lens portions) 112a, 112b, and 112c, which have different optical axes. In FIG. 14, by providing a plurality of optical axes to the projection lens 112, projection ranges, the number of which is equal to that of optical axes, are obtained.
However, since a projection system (projection device) that illuminates the pattern 83 with the light source 84 forms the image of the projection pattern 83 on the object, the illumination range corresponds to a blurred image of the light source 84. Accordingly, when the plurality of optical axes are provided like the projection lens 112 shown in FIG. 14, and the lens is divided to attain a size reduction of the overall device, the illumination range as the blurred image of the light source 84 is also divided, resulting in narrow illumination ranges.
To avoid such problem, the projection lens may be made up by coupling a plurality of lenses without dividing a single lens. However, such arrangement results in a huge device.
In order to solve the above-mentioned problems, a projection system for automatic focus detection shown in FIG. 15 has been proposed conventionally. FIG. 15 is a schematic view showing the principal part of a conventional projection system for automatic focus detection.
Referring to FIG. 15, a light source unit 124 itself, which integrates a light source and a projection pattern is imaged and projected onto an object by a projection lens 122. With this arrangement, the illumination range is not divided by the divided lens portions unlike in the projection system shown in FIG. 14.
However, in the projection system shown in FIG. 15, since off-axis projection lenses 122a and 122c are considerably decentered from the optical axis to broaden the projection range, the projection lens becomes bulky, and it is very difficult to maintain high projection/imaging performance of the lens up to the edge portions of the field angle. Consequently, poor off-axis projection performance adversely affects focus detection precision.
In all the prior arts described above, when the pattern is also to be divisionally projected in a direction perpendicular to the plane of drawing used in their descriptions, the number of light sources and projection lens portions must be increased as in division in the up-and-down direction on the plane of drawing. As a result, the entire device becomes larger in proportion to the illumination range.